This invention relates to improving the properties of a unique bismaleimide resin which has been designated APO-BMI. This resin is disclosed and claimed in U.S. Pat. No. 4,464,520. It was directed to solving a difficulty relating to the processability of conventional bismaleimide resins which have a tendency to begin to polymerize only slightly above their melting temperature. Consequently, application of such resins has been difficult. The inventors of the '520 patent discovered that by reacting maleic anhydride with certain specific aromatic amines which were substituted in the ortho position they were able to create a unique family of bismaleimide resins which could be melted and held as a melt for an extended period without undergoing polymerization. The advantages of such compounds are obvious; however, they are still subject to disadvantages in that the resulting resins are rather brittle and, consequently, some improvement of such resins has been sought.
Functional chain extension of bismaleimides by Michael addition is well known as exemplified by the commercially available Kerimid 601 resin. However, such additions were not expected to be easily accomplished with the unique APO-BMI resins of the '520 patent since they are considered to be sterically hindered, thus decreasing the reactivity of the maleimide double bond. The present inventors have found that such materials may be chain extended by a reaction with certain diamines to produce significantly improved properties in the finished bismaleimide resins.